1. Field of the Invention
The present invention relates to modified polyolefins with an exceptional profile of properties, the preparation of which is based on semicrystalline polyolefins with syndiotactic structural elements, to a process for preparation thereof and to the use thereof, especially as an adhesive or as a constituent of adhesives.
2. Description of the Related Art
Amorphous poly-alpha-olefins serve in many cases as adhesive raw materials for a wide range of applications. The field of use extends from the hygiene sector through laminations and packaging adhesives as far as construction adhesives and uses in wood processing. Unmodified amorphous poly-alpha-olefins (known as APAOs) are notable for purely physical curing, which is reversible as desired owing to the thermoplastic character thereof. However, they have only limited tensile strengths and adhesive shear strengths, and a relatively low thermal stability. Moreover, they cannot be used to achieve covalent incorporation of reactive surface groups (for example —OH) into an adhesive bond.
The described disadvantages of unmodified APAOs can be remedied by a subsequent functionalization (modification), for which carboxylic acids or carboxylic acid derivatives and/or silanes in particular can be used for modification.
The preparation of silane-modified polyolefins by reaction of polyethylene with unsaturated silanes has been known for sometime. EP 0004034 gives an early description of a method for crosslinking poly(α-olefins) with the aid of silane bonds, the intention being to achieve maximum degrees of crosslinking. The crosslinking directly follows the grafting and leads to stiff, high-strength materials with low embrittlement temperature, as used, for example, for the production of cable sheathing and/or mouldings. The polymers described cannot be used as adhesives.
DE 1963571, DE 2353783 and DE 2406844 describe processes for crosslinking polyethylene polymers or ethylene copolymers which contain small amounts of propene and/or 1-butene. The target products are crosslinked mouldings based on polyethylene.
DE 2554525 and DE 2642927 describe processes for producing extruded products, including the silane functionalization of a polymer, the incorporation of a silanol condensation catalyst, and the shaping and crosslinking of the polymer, in one operation, by using an extruder. The end applications mentioned are cables, pipes and hoses. Adhesive bonds are not possible with the polymers produced in this way, and further processing overall is possible only to a very limited degree owing to the crosslinking performed immediately after the modification.
It has likewise been known for sometime that it is possible to improve the adhesion of polyolefins to functional surfaces, for example glass, by the introduction of silane groups. For instance, U.S. Pat. No. 3,075,948 gives an early description of graft polymers consisting of unsaturated silane monomers and solid poly(alpha-olefins) having 2-6 carbon atoms, which are said to have improved heat resistance and good adhesion to glass. The resulting modified polymers are used for the production of mouldings and containers, and as a coating for glass vessels; they are not suitable for use as melt-applied adhesives owing to the completely different profile of requirements (melt viscosity, material stiffness in the uncrosslinked state, etc.).
The use of amorphous poly(alpha-olefins) for silane crosslinking is also already known. For example, EP 0260103 describes amorphous silane-modified polymers with a saturated carbon skeleton and low molecular weight, which are used as coating materials for protection from weathering influences. Examples of such polymers include copolymers of ethylene and/or α-olefins, especially EPM and EPDM. The base polymers described are amorphous and rubber-like, and have a high elasticity. Owing to their rubber-like character, proccessability in the uncrosslinked state is poor. The products are unsuitable for the intended applications in the adhesives and sealants sector in the present application.
DE 4000695 describes the use of substantially amorphous poly(alpha-olefins) in a process in which the APAOs are reacted with a free-radical donor and optionally additionally graftable monomers (e.g. vinylsilanes) under simultaneous shear stress. The resulting products are suitable for use as carpet coating materials or as melt-applied adhesives. The substantially amorphous polyolefins used to prepare the modified poly(alpha-olefins), however, themselves (i.e. in the unmodified state) have only poor to moderate material or adhesive properties, such that the modified polymers are also suitable only for applications with low requirements. More particularly, the unmodified, predominantly amorphous polyolefins have a high polydispersity, which leads to disadvantages in material cohesion, and to problems with outgassing low molecular weight constituents. The microstructure of the polymer chains is also not very well-defined, one reason being the heterogeneous polymerization catalysts used to prepare the unmodified polyolefins, and so controlled adjustment to particular material or adhesive requirements is possible only with difficulty. An additional factor is that the modified polymers possess only low functionalization, since the ratio of graft polymerization to chain cleavage is unfavourable. Owing to the low functionalization, the crosslinking reaction proceeds slowly; attachment to reactive surfaces is only relatively weak. An additional factor is that the tensile strength both of the uncrosslinked and of the crosslinked modified polyolefin reaches only relatively low values, as a result of which the products remain excluded from many areas of application.
In JP 2003-002930A, graft polymers are prepared from amorphous poly(α-olefin)s, unsaturated carboxylic acids and optionally additionally unsaturated aromatic substances (e.g. styrene). The polyolefins used are amorphous and do not have a crystallinity of >1 J/g in DSC measurements. Moisture-crosslinking monomer systems, for example vinylsilanes, are not discussed; the grafted polyolefins do not have the desired material parameters owing to the properties of the base polymer thereof and the graft monomers used; more particularly, they are too soft, have only a low heat resistance and exhibit too low a tensile strength.
WO 03/070786 describes a process for preparing modified poly(1-butene) polymers, the modified poly(1-butene) polymers obtainable therefrom, and an adhesive composition comprising the modified poly(1-butene) polymers. The poly(1-butene) base polymer used for the modification has a melting point in the range from 0 to 100° C., an isotacticity index of <20% and a polydispersity of <4.0. The graft monomers mentioned are unsaturated carboxylic acids, carboxylic anhydrides, or corresponding derivatives such as amides, esters, etc.
Moisture-crosslinking monomers, for example vinylsilanes, are not described. The modified polymers prepared are relatively soft and of relatively waxy nature owing to their low crystallinity. The low melting point causes poor heat resistance of the adhesive bonds. The polymers are unsuitable for the applications intended in the present application.
WO 2006/069205 describes modified polyolefins based on low-viscosity polypropylene polymers with a propylene content of >50 mol % and a proportion of isotactic propylene triads of >85%, which can be prepared, among other methods, by a free-radical graft polymerization. Owing to the material properties of the base polymers used, the products obtained are unsuitable for the fields of use intended in the present application.
WO 2007/067243 describes polypropylene polymers functionalized by carboxylic acids and having a high to very high propylene content (75-90 mol %), which are prepared on the basis of propylene-based homo- and/or copolymers with a weight-average molar mass of <100 000 g/mol, a melting point of <157° C. and a melt viscosity at 190° C. of <40 000 cPs at reaction temperatures of 130-165° C. Moisture-crosslinking systems, for example based on silanes, are not described. Owing to the base polymers used and the graft monomers used, the products described are unsuitable for the fields of use intended in the present application.
WO 91/06580 describes silane-modified unsaturated amorphous polymers which can be used in the crosslinked state, for example as mouldings. Further use examples of the silane-modified polymers include adhesive compositions, including melt-applied adhesives. Examples of unsaturated base polymers include rubber-like polymers, for example styrene-butadiene block copolymers (SBS), styrene-isoprene block copolymers (SIS), styrene-butadiene rubber (SBR), nitrile rubber, polychloroprene rubber and butyl rubber. All base polymers mentioned have rubber elasticity (i.e. also poor proccessability) and/or other adverse material properties (for example poor heat resistances), which make them unsuitable for melt-applied adhesive applications.
The use of silane-modified polymers in hotmelt adhesives is likewise known. For example, WO 89/11513 describes an adhesive composition which contains at least one silane-modified or silane-grafted semicrystalline polymer. The base polymers mentioned are especially homo-, co- and terpolymers of C2-6-α-olefins, and also isotactic polypropylene polymers and blends of polypropylenes, especially when they also contain atactic polypropylene. The graft reaction proceeds at temperatures of 140 to 250° C. Atactic polypropylene without defined polymer microstructure intrinsically has a very low softening point [see, for example: H.-G. Elias; Makromoleküle [Macromolecules]; Vol. III; Wiley-VCH: Weinheim; 2001]. The procedure described in WO 89/11513 leads to products with unsatisfactory material properties, especially with regard to cohesion, adhesion (adhesive shear strength) and heat resistance in the uncrosslinked state (for example immediately after application). The adjustment of the viscosity, melting behaviour and tack of the adhesive composition is attributed causally to the use of relatively long-chain silane monomers (≧3 connecting atoms between silicon atom and the polymer chain), which are said to lead to a “more open structure”. The use of relatively long-chain silane monomers is disadvantageous in that it leads to weaker crosslinking as a result of a higher degree of polymerization of the network chains (i.e. of the monomeric base units between two crosslinking sites), which additionally has an adverse effect on the material properties of the graft polymer.
DE 19516457 describes a crosslinkable adhesive composition consisting of at least 50% by mass of a silane-grafted polyolefin and additionally of a carboxylic acid-grafted polyolefin. The base polymers specified for the grafting are poly(ethylene-co-vinyl acetate), polypropylene, polyethylene, poly(ethylene-co-methacrylate) and poly(ethylene-co-methacrylic acid). Owing to the base polymers used and the graft monomers used, the products described are unsuitable for the desired fields of use.
EP 1508579 describes (silane-)modified crystalline polyolefin waxes with a high propylene content. Owing to their wax-like properties and the resulting poor adhesive properties, the polymers described are unsuitable for the fields of use intended. High functionalization according to the present requirements is not achievable owing to the material properties of the base polymers used.
WO 2007/001694 describes adhesive compositions which contain functionalized polymers (preferably maleic anhydride-grafted propylene polymers). The base polymers used are propylene(co)polymers with high isotactic contents (>75% isotactic triads) and a polydispersity of 1.5 to 40, i.e. predominantly crystalline polymers which possess a very broad molar mass distribution, as normally achievable only in polymers with multimodal distribution.
WO 2007/002177 describes adhesive compositions based on poly(propylene) random copolymers, functionalized polyolefin copolymers which are rich in syndiotactic units, and non-functionalized adhesive resins, the poly(propylene) random copolymers having an enthalpy of fusion of 0.5 to 70 J/g and a proportion of isotactic propylene triads of at least 75% (more preferably >90%), and the functionalized (syndiotactic) polymers used having a content of functional monomer units of at least 0.1% and being present with a proportion of <4% by mass in the adhesive composition. The poly(propylene) random copolymers described have a polydispersity of 1.5 to 40, which indicates a multimodal molar mass distribution and the simultaneous presence of a plurality of catalyst species. Polymers with a very broad molar mass distribution, especially with a molar mass distribution of >5, exhibit a very inhomogeneous distribution of functional groups on the polymer chains in free-radically initiated graft reactions.
WO 2007008765 describes the use of low-viscosity silane-grafted poly(ethylene-co-1-olefin) polymers as an adhesive raw material. The polymers used for modification have an ethylene content of at least 50 mol % of ethylene. The silane-grafted polymers have very low failure temperatures. The use of polyolefins with a high ethylene content inevitably means the presence of long ethylene blocks in the polymer. This in turn leads to poor wetting and adhesive properties on many plastics surfaces, such that very many adhesion problems cannot be solved in an optimal manner. In addition, long polyethylene sequences tend to peroxidic crosslinking (which is exploited industrially in the production of cable sheathing among other applications), as a result of which gel formation is unavoidable.
EP 0827994 describes the use of silane-grafted amorphous poly(alpha-olefins) as a moisture-crosslinking adhesive raw material or adhesive. The base polymers used are atactic polypropylene (aPP), atactic poly(1-butene), or preferably co- or terpolymers formed from C4-C10 alpha-olefins (0-95% by mass), propene (5-100% by mass) and ethylene (0-20% by mass). The silane-modified APAO described in the examples has a softening point of 98° C., a needle penetration of 15*0.1 mm and a melt viscosity of 6000 mPa*s. The atactic polyolefins and APAOs used have a relatively low molar mass and a relatively low crystallinity, which leads on modification to products with low flexibility, which possess a low functionality and a low tensile strength, and are therefore unsuitable for many applications.
The use of metallocene compounds as a catalyst in olefin polymerization has likewise been known for some time. Kaminsky et al. have shown that the cyclopentadienylzirconium dichloride/methylaluminoxane (Cp2ZrCl2/MAO) catalyst system is very suitable for polymerization (Adv. Organomet. Chem. 1980, 18, 99-149). Since this time, the use of metallocene compounds in conjunction with methylaluminoxane (MAO) has become widespread in polymerization reactions. For instance, there is a multitude of publications concerned with metallocene-catalysed olefin polymerization, for example of propene, for example U.S. Pat. No. 6,121,377, EP 584 609, EP 516 018, WO 2000/037514, WO 2001/46274 and US 2004/0110910.
In the polymerization of propene or higher homologues thereof, different relative stereoisomers may be formed. The regularity with which the configurative repeat units follow one another in the main chain of a macromolecule is referred to as tacticity. To determine the tacticity, the monomer units of a polymer chain are considered and the relative configuration of each (pseudo)asymmetric chain atom relative to the preceding atom is determined. Isotacticity refers to the situation where the relative configuration of all (pseudo)asymmetric chain atoms found is always the same, i.e. the chain is formed from only one single configurative repeat unit. Syndiotacticity, in contrast, refers to the situation where the relative configurations of successive (pseudo)asymmetric chain atoms are the opposite of one another, i.e. the chain is formed from two different alternating configurative repeat units. In atactic polymers, finally, the different configurative repeat units are arranged randomly along the chain.
The physical properties of propylene polymers depend primarily on the structure of the macromolecules and hence also on the crystallinity, the molecular weight thereof and the molecular weight distribution, and can be influenced by the polymerization process used and especially the polymerization catalyst used [R. Vieweg, A. Schley, A. Schwarz (eds.); Kunststoff Handbuch [Plastics Handbook]; vol. IV/“Polyolefine” [Polyolefins]; C. Hanser Verlag, Munich 1969].
Polypropylene polymers are thus divided into atactic, isotactic and syndiotactic polymers on the basis of their tacticity. Additional special forms include the so-called hemiisotactic polypropylene polymers and the so-called stereoblock polymers. The latter are usually polymers with isotactic and atactic stereoblocks which behave like thermoplastic elastomers, since a physical crosslinking of the polymer chains takes place, which leads to a connection of different crystalline polymer regions (A. F. Mason, G. W. Coates in: “Macromolecular Engineering”; Wiley-VCH, Weinheim; 2007).
Atactic polypropylene has a low softening point, a low density and a good solubility in organic solvents. Conventional atactic polypropylene (aPP) features a very wide molecular weight distribution, which firstly leads to a broad melting range, and secondly entails high low molecular weight fractions which have a greater or lesser tendency to migrate. aPP has a very low tensile strength of approx. 1 MPa, but on the other hand has a very high elongation at break of up to 2000% (H.-G. Elias; Makromoleküle; vol. III; Wiley-VCH; Weinheim; 2001). Owing to the low softening point, the thermal stability of aPP formulations is correspondingly low, which leads to a significant limitation in the area of use. Purely atactic polypropylene polymers can also be prepared by metallocene catalysis to obtain either very low molecular weight or relatively high molecular weight polymers (L. Resconi in: “Metallocene based Polyolefins”; J. Scheirs, W. Kaminsky (eds.); J. Wiley & Sons; Weinheim; 1999).
Syndiotactic polypropylene is highly transparent and is notable for good thermal stability, the melting temperature being below that of isotactic polypropylene. It has high fracture resistances coupled with moderate elongation at break (A. F. Mason, G. W. Coates in “Macromolecular Engineering”; Wiley-VCH, Weinheim; 2007). A disadvantage is the slow crystallization from the melt which is observed in many cases. Owing to physical loops, the melt viscosity of syndiotactic polypropylene with comparable molar mass is significantly higher than that of isotactic polypropylene, i.e. it is possible to achieve the same melt viscosity with significantly lower molar masses. Syndiotactic and isotactic polypropylene are immiscible from a certain molar mass; corresponding polymer blends tend to phase separation. Polymer blends of syndiotactic polypropylene with other polyolefins exhibit a significantly higher elongation at break than blends comprising isotactic polypropylene (T. Shiomura, N. Uchikawa, T. Asanuma, R. Sugimoto, I. Fujio, S. Kimura, S. Harima, M. Akiyama, M. Kohno, N. Inoue in: “Metallocene based Polyolefins”; J. Scheirs, W. Kaminsky (eds.); J. Wiley & Sons; Weinheim; 1999). Conventional heterogeneous Ziegler-Natta catalysts are incapable of preparing syndiotactic polypropylene.
Isotactic polypropylene features a high melting temperature and good tensile strength. For 100% isotactic polypropylene, the calculated melting temperature is 185° C. and the melting enthalpy is approx. 207 J/g (J. Bicerano; J. M. S.; Rev. Macromol. Chem. Phys.; C38 (1998); 391ff). As a homopolymer, however, it has a relatively low cold stability and a high brittleness, and a poor heatsealability or weldability. The tensile strength (fracture) is approx. 30 MPa, and virtually no elongation at break occurs. Improved material properties can be established by co- or terpolymerization with ethylene and 1-butene, the comonomer content for copolymers with ethylene being typically <8% by mass and, for terpolymers with ethylene and 1-butene, <12% by mass (H.-G. Elias; Makromoleküle; vol. III; Wiley-VCH; Weinheim; 2001). At the same MFR (melt flow rate), isotactic polypropylene which has been prepared by conventional heterogeneous Ziegler-Natta catalysis has a significantly lower intrinsic viscosity than polypropylene which has been prepared by metallocene catalysis. The impact resistance of the metallocene-based polymer is above that of the Ziegler-Natta material within a wide molar mass range. The proportion of xylene-soluble constituents is typically significantly <1% by mass for isotactic poly(propylene) homopolymer which has been obtained by metallocene catalysis; in the case of random copolymers with ethylene, according to the ethylene content, xylene-soluble fractions of not more than 5% by mass are found (W. Spaleck in: “Metallocene based Polyolefins”; J. Scheirs, W. Kaminsky (eds.); J. Wiley & Sons; Weinheim; 1999).
Since the solubility of polypropylene depends both on the molecular weight and on its crystallinity, a corresponding fractionation can be effected by means of dissolution tests [A. Lehtinen; Macromol. Chem. Phys.; 195(1994); 1539ff].
With regard to the solubility of polypropylene polymers in aromatic solvents and/or ethers, there are numerous publications in the scientific literature. For example, it has been found that the proportion of xylene-soluble constituents is typically significantly <1% by mass for isotactic poly(propylene) homopolymer which has been obtained by metallocene catalysis; in the case of random copolymers with ethylene, according to the ethylene content, xylene-soluble fractions of not more than 5% by mass are found (W. Spaleck in: “Metallocene based Polyolefins”; J. Scheirs, W. Kaminsky (eds.); J. Wiley & Sons; Weinheim; 1999).
It has been known for some time that it is possible by means of extraction with ethers to obtain amorphous atactic fractions [J. Boor; “Ziegler-Natta Catalysts and Polymerization”; Academic Press; New York; 1979] and low molecular weight fractions with low crystallinity [G. Natta, I. Pasquon, A. Zambelli, G. Gatti; Makromol. Chem.; 70 (1964); 191ff] from polypropylene polymers. Highly crystalline isotactic polymers, in contrast, have a very low solubility both in aliphatic solvents and in ethers, specifically also at elevated temperature [B. A. Krentsel, Y. V. Kissin, V. I. Kleiner, L. L. Stotskaya; “Polymers and Copolymers of higher 1-Olefins”; p. 19/20; Hanser Publ.; Munich; 1997]. The soluble polymer fractions generally have only a very low crystallinity, if any, and do not exhibit a melting point [Y. V. Kissin; “Isospecific polymerization of olefins”; Springer Verlag; New York; 1985]. Tetrahydrofuran-soluble polypropylene oligomers have very low number-average molar masses of significantly less than 1500 g/mol [H. El Mansouri, N. Yagoubi, D. Scholler, A. Feigenbaum, D. Ferrier; J. Appl. Polym. Sci.; 71 (1999); 371ff].
The different polymer types differ significantly in their material properties. The crystallinity of highly isotactic or syndiotactic polymers is very high owing to their high order. Atactic polymers, in contrast, have a high amorphous content and accordingly a low crystallinity. Polymers with high crystallinity exhibit many material properties which are undesired especially in the field of hotmelt adhesives. For example, a high crystallinity in low molecular weight polymers leads to very rapid crystallization with open times (“open time”=time interval within which the parts to be adhesive bonded can be bonded to one another) of in some cases less than one second. In the case of application (for example in the case of nozzle application by spraying), this leads to blockage of the application equipment used even in the event of very small temperature variations, and hence to very poor process stability. An additional factor is the exceptionally short time interval within which the adhesive bond can be joined after the application. Highly crystalline polymers at room temperature are additionally hard and brittle and have only a very low flexibility, which is likewise undesired in the case of adhesive bonds. An additional factor is that very high amounts of energy are required for the melting of highly crystalline polymers at individual points (at the site of introduction) and over the entire conduit system, which, as well as economic effects, also has adverse effects for proccessability. Furthermore, in the case of highly crystalline polymers, there is spontaneous (immediate) solidification below the melting point (which, in analysis by means of differential calorimetry (DSC), is characterized by a sharp melting peak in the 2nd heating), which makes impossible or greatly complicates the proccessability of such polymers or of the products produced on the basis of such polymers.
The preparation of poly-1-olefins with syndiotactic propylene segments by use of metallocene catalysts is known.
For example, U.S. Pat. No. 5,476,914 describes highly crystalline polypropylene polymers. Owing to their high crystallinity, the polymers described, however, are unsuitable for the fields of use intended in the present application.
EP 351391 describes highly crystalline polypropylene homopolymers which have a polydispersity of >3 and a syndiotacticity index of at least 0.8, and a specific linkage of the syndiotactic triads. The polymers are prepared by means of metallocene catalysts with an unsymmetric ligand structure (for example with dimethylmethylene(cyclopentadienyl)(fluorenyl)ZrCl2). The polymers described have very high crystallinity and are therefore unsuitable for the applications intended in the present application.
EP 0747406B1 addresses the topic, but the cyclopentadienyl radical on the metallocene here must always contain a sterically voluminous substituent (at least of the size of a trimethylsilyl group), which likewise leads to products with high tacticity owing to the greatly limited space in the vicinity of the polymerization site.
EP 351392 describes bridged metallocenes (for example dimethylmethylene(cyclopentadienyl)(fluorenyl)ZrCl2) which are used to prepare highly crystalline, syndiotactic polypropylene polymers. It is explicitly pointed out that the preparation of polymers with atactic units is unfavourable.
EP 354893 describes a process for preparing symmetric, free-flowing polypropylene particles of “particular size” and high density using metallocene catalysts. One metallocene catalyst mentioned is dimethylmethylene(cyclopentadienyl)(fluorenyl)ZrCl2. The polymers prepared in the manner described by slurry polymerization do not have the material properties required of adhesive raw materials.
JP 02173104 describes the preparation of polyolefins with narrow molecular weight distribution based on microparticle-supported metallocene catalysts (for example dimethylmethylene(cyclopentadienyl)(fluorenyl)ZrCl2). As a result of the use of solid support components, the polymerization reaction is performed as a heterogeneous polymerization, the results of which are not comparable to those of a homogeneous reaction regime (according to the present patent application). The described polymers with high density and high crystallinity do not have the material properties required of adhesive raw materials.
For instance, EP 0384264 describes distribution based on microparticle-supported metallocene catalysts (for example dimethylmethylene(cyclopentadienyl)-(fluorenyl)ZrCl2, for the preparation of polypropylene waxes. Owing to their high crystallinity and their hardness, the isotactic crystalline waxes described are unsuitable for the uses intended in the present application.
JP 02300212 describes the preparation of syndiotactic polypropylene polymers with the aid of a metallocene catalyst with unsymmetric ligand structure, for example dimethylmethylene(cyclopentadienyl)(fluorenyl)ZrCl2 in which a preliminary polymerization is effected in an aromatic solvent, and the slurry formed there is utilized for the further polymerization. Adhesive raw material polymers cannot be obtained in this way.
JP 03000709 describes the preparation of syndiotactic polypropylene using metallocene catalysts with an unsymmetric ligand structure, for example dimethylmethylene(cyclopentadienyl)(fluorenyl)ZrCl2, in which the metallocene complex is applied to an inorganic silica support. This affords highly syndiotactic systems with high crystallinity, which are unsuitable for use as an adhesive raw material.
JP 03066710 describes the preparation of syndiotactic polypropylene of high density using metallocene catalysts with an unsymmetric ligand structure. As a result of the use of inorganic supports, the polymerization reaction is performed as a heterogeneous polymerization, the results of which are not comparable to those of a homogeneous reaction regime. The described polymers with high density and high crystallinity do not have the material properties required of adhesive raw materials.
EP 423101 describes the use of isopropylidene(3-methylcyclopentadienyl-1-fluorenyl)zirconium dichloride for preparation of polypropylenes with hemiisotactic structure. Explicit reference is made to the necessity of absence of bilateral symmetries in the metallocene molecule. Mixtures of hemiisotactic and syndiotactic or isotactic polyolefins are obtained only by using two different metallocene catalysts.
Ewen et al. have shown that, by propene homopolymerization with isopropylidene(Cp)(Flu)ZrCl2 or the analogous Hf compound with MAO as a cocatalyst, it is possible to obtain syndiotactic polypropylene polymers with high stereospecificity, which contain virtually no atactic components [J. A. Ewen, R. L. Jones, A. Razavi; J. Am. Chem. Soc.; 110(1988); 6255/56].
U.S. Pat. No. 4,892,851 describes metallocene catalysts for preparation of syndiotactic polyolefins, especially of syndiotactic polypropylene, and a process for preparing said polymer using the metallocenes claimed. The examples show that exclusively propylene is used; the establishment of specific polymer properties by the use of comonomers is not described. It is pointed out explicitly that it is unfavourable to prepare all three stereospecific polymer types, and that preference is given to those catalysts which prepare predominantly isotactic and/or syndiotactic polymer. The metallocene catalysts mentioned prepare polymers with a high syndiotacticity index. Polymers with very high stereo specificity are unsuitable for use as adhesive raw materials owing to their material properties, more particularly their inadequate adhesion and their hardness.
DE 3907965 describes a process for preparing syndiotactic polyolefins by means of metallocene catalysis using an aluminoxane cocatalyst, the outstanding features of the polyolefin emphasized being a high molar mass, a narrow molar mass distribution and a very high syndiotacity. The objective mentioned is a syndiotacity of >90%. The polymers described are unsuitable for the use as adhesive raw materials intended in the present application.
EP 433987 describes high molecular weight polypropylene copolymers with high syndiotacity of the polypropylene component for the production of slabs, moulding materials, packaging films and coatings. The metallocenes mentioned include diphenylmethylene(9-fluorenyl)(cyclopentadienyl)ZrCl2 and dimethylmethylene-(cyclopentadienyl)(fluorenyl)ZrCl2. Adhesive raw material polymers are not obtained in the manner described.
EP 480390 describes a process for preparing polyolefins with high tacticity and large molar mass. The examples show that both isotactic and syndiotactic polymers are obtained, the isotacticity indices of which are in the range from 90 to 99%, and the syndiotacticity indices of which are 96%. Polymers with high atactic components and polymers with a specific ratio of syndiotactic to isotactic and atactic structural elements are not described.
U.S. Pat. No. 5,459,117 describes specific metallocene catalysts and the use thereof, particular emphasis being given to the fact that the isotactic and syndiotactic polymers preparable with the metallocene catalysts described are preparable without stereo defects, i.e. have a very high tacticity and hence also crystallinity. An identifying feature mentioned for the isotactic and syndiotactic polyolefins is the insolubility thereof in cold xylene. Polymerization catalysts which prepare either syndiotactic or isotactic polymer without atactic components and/or stereo defects are referred to as particularly desirable.
GB 2310398 and DE 19707034 describe heat-activatable labels with an adhesive layer based on ethylene-1-olefin copolymers, which are prepared by metallocene catalyst. Owing to their high ethylene content, the polymers described are unsuitable for the graft modification described in the present application.
WO 99/20664 describes the preparation of polypropylene homopolymers referred to as “hybrid polymers” using metallocene catalysts. It becomes clear in the examples that the polymers obtained have a very high isotactic polymer content.
WO 2000/037514 describes branched semicrystalline poly(ethylene-co-propylene) copolymers, which are said to be suitable, inter alia, for use in hotmelt adhesives. Polymers which have a high proportion of branches, however, tend to gelate in the course of free-radical graft modification, and they are therefore unsuitable for applications intended in the present application.
Hopf and Kaminsky obtained polymers with a very high syndiotacticity index in the homopolymerization of propene with the diphenylmethylene(2,7-ditertbutylfluorenyl)(cyclopentadienyl)ZrCl2-MAO system. [A. Hopf, W. Kaminsky; Catal. Commun.; 3 (2002), 459-464.]
Elsewhere, Kaminsky et al. obtained highly syndiotactic propene homopolymers in the study of the polymerization behaviour of CS-symmetric zirconocenes using [(p-OMePh)2C-(Cp)(2,7-ditertbutyl-Flu)]ZrCl2 or [Ph2C(Cp)(Flu)]ZrCl2 [Kaminsky, Walther; Hopf, Andreas; Arndt-Rosenau, Michael; Macromolecular Symposia (2003), 301-307.]
Seraidaris et al. describe the homopolymerization of propene and the copolymerization of propene with low proportions of ethylene using metallocenes/borate and metallocene-MAO catalyst systems. In the case of use of [(p-OMePh)2C-(Cp)(2,7-ditertbutyl-Flu)]ZrCl2, purely amorphous products were obtained even at low ethylene contents. [Seraidaris, Tanja; Kaminsky, Walther; Seppaelae, Jukka; Loefgren, Barbro; Macromolecular Chemistry and Physics (2005), 206(13), 1319-1325].
Highly isotactic or syndiotactic polypropylene homo- or copolymers with ethylene and/or higher olefins, as described in the publications cited, are unsuitable for use as a melt-applied adhesive or adhesive raw material.
There was therefore a need for functionalized (semicrystalline) polyolefins with improved material properties, especially in functionalized polyolefins which are prepared on the basis of unfunctionalized polyolefins with a defined polymer structure, which enable a high degree of functionalization with simultaneously low polymer degradation owing to their defined chain structure in the free-radical functionalization. At the same time, the functionalized polyolefins should additionally exhibit high flexibility and high transparency coupled with good cohesive and adhesive properties, without having the disadvantages of highly isotactic or highly syndiotactic polymer systems.